Austria has denied the United States permission to use its airspace for military operations against Iran, citing the country's neutrality law, the defence ministry said on April 2.

A ministry spokesperson confirmed there had been "several" requests from Washington but declined to specify the number, Austrian public broadcaster ORF reported.  Each case would be assessed individually in coordination with the foreign ministry.

Austria has not imposed a blanket ban on US overflights but is reviewing requests on a case-by-case basis. The country has maintained a policy of military neutrality since 1955.

The opposition Social Democratic Party (SPO) called on the government to go further. "Defence Minister Klaudia Tanner should not approve a single further US military flight to the Gulf.

Nor should she approve any transport flights or other logistical support," said Sven Hergovich, SPO leader in Lower Austria.

"This war is damaging Austrian economic interests, Europe as a whole, and world peace."

Austria is now the fourth European country to restrict US military access over the Iran war. Spain closed its airspace to military flights related to the conflict and refused access to two bases.

Italy secretly blocked two US F-15 fighter jets from landing at its Sigonella base in Sicily after Washington attempted to use the facility without prior authorisation. Switzerland has also restricted overflights.

The growing European resistance to the war has strained transatlantic relations. Trump said on April 1 he was "absolutely" considering withdrawing from NATO, accusing allied nations of treating America "very badly."

US Secretary of State Marco Rubio expressed frustration at Spain's public refusal, with some in Washington calling for sanctions against Madrid.